After the end of World War I, the Treaty of Versailles stipulated that Germany should not build submarines and should not launch unrestricted submarine warfare. However, on the one hand, Germany secretly studied submarines in China, and on the other hand, it ordered eight submarines from abroad. On march 16,1935, germany openly tore up the treaty of versailles; just three months later, germany's first post-war submarine, the U-1, was launched on june 15. By september of this year, germany had built a total of nine submarines and set up a submarine force, the "wedigen" submarine fleet, under the command of dunnitz of the be very famous. By the end of 1935, the German submarine force had already